Query about 19th Century Bridge, Western Heights, Dover

#1
The Napoleonic defenses on Dover's Western Heights consists of a citadel, bastions and redoubts originally surrounded by over 4 miles of brick-lined 'ditches' (moats).

The North Entrance to the fortifications consists of two bridge combinations and a road tunnel.

My specific question concerns the 'outer bridge' which I've been told is a 'drop down' bridge, originally pivoting on the brick arch you can see towards the far end of the bridge (by the way, the I-beams replaced the original structure during WWII):

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[Click on the photo to see the original web-page][/align]

Apparently, the bridge dropped to the moat floor at the near end in the photo with the far end rising up into the air. Does this sound likely?

NB There are two pulleys recessed into the short brick columns on either side of the far end of the bridge.

I've recently made two videos of the North Entrance: the first video is about the associated tunnel and the second one shows the bridges themselves.

This is a view taken from beneath the brick arch of the outer bridge:

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[Click on the photo to see the original web-page][/align]

I would be grateful for any help! :)

John Latter / Jorolat

PS There's an 'insert video' tag in the text editor, but I couldn't get it to work - does anyone know how?

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#2
Interesting post, I love this kind of stuff. Your drop-down bridge certainly sounds possible, but perhaps a bit complicated.

If I were you I'd look for evidence on the near side of the bridge. There must have been some sort of mechanism in place there to hold the bridge up when it was in the horizontal position.

Regards
T_T
 
#3
Tartan_Terrier said:
Interesting post, I love this kind of stuff. Your drop-down bridge certainly sounds possible, but perhaps a bit complicated.

If I were you I'd look for evidence on the near side of the bridge. There must have been some sort of mechanism in place there to hold the bridge up when it was in the horizontal position.

Regards
T_T
Thank you for the quick reply and comments, Tartan_Terrier :)

I don't know if you've watched the second video, so here's the only still photo of the inner bridge that I have:
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[Click on the photo to see the original web-page][/align]

On the left there are the upright beams of a drawbridge of which only half is visible: behind the brick-work there is a winch for operating the cables and two enormous counterweights.

This drawbridge, along with nearby smaller underground ones, is a perfectly balanced machine requiring the minimum of effort to operate.

By way of contrast, the 'drop-down' outer bridge appears to be quite clumsy in how it works - and I'm also intrigued by the presence of the two pulleys recessed into the brickwork on the far/tenaille end of the bridge.

As far as I remember, the pulleys are about 6 inches in diameter and 2-3 inches wide - they seem to be more appropriate for part of the bridge, rather than the whole of it.

If the whole of the outer bridge tilted in the manner described, then I feel pretty sure an 'elegant' solution would have been found to solve the problem of raising it again.

I didn't know the whole bridge was supposed to tilt when I took the video so I didn't pay much attention to the wall on the near end of the bridge at the time.

Once I found out how it was supposed to work, however, I checked the second video again: starting around the 5 min 50 secs mark, there's a sequence showing the near wall, but all I can see is a narrow vertical recess about 3 or 4 feet down from the bridge level.

The photo in the 1st post was taken about 2 years ago and shows a 'line' running along the top of the brick arch, but it's missing in the video.

Anyway, I'm taking some people up there this Sunday (weather permitting) so I'll have a closer look at the near wall, but I probably won't have time to climb up onto the tenaille to look at the top of the arch on this occasion.

Thanks again!

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#4
I`ve spent many a day out wandering around the Plymouth forts so your videos were intriguing, I`ve just spent an hour pondering them and can`t see how it would be practical for such a design.
If there were supports on the outer wall they would get in the way when the bridge was dropped. Without supports there would be a tremendous stress on the 2/3 of the bridge which was hanging in thin air especially at the stone arch/fulcrum point.
The other 1/3 would`ve needed counter weights underneath it to aid levelling the bridge after it was dropped, and pulleys would be needed below the level of the bridge with ropes going vertical to the underside of the 1/3 in order to pull down on it so raising the 2/3 section.
In theory I can understand the idea, its far quicker to use gravity to drop a bridge into a ditch than it is to manually wind up a drawbridge, to deny access to an attacking force. But if the attackers managed to get into the ditch, the design provides an access ramp albeit at around 30-40 degrees, up which they could clamber, drop down 12-15 feet at the far end and they`re inside the fort. Difficult, but a lot easier than trying to climb vertical walls.
It might be worth asking the person who told you it was a drop down bridge where they got the idea, if its bonafide then there must be some sort of documented evidence as to the design.
Could you keep us informed of any progress you make on this please ? :salut:
 
#5
slick said:
I`ve spent many a day out wandering around the Plymouth forts so your videos were intriguing, I`ve just spent an hour pondering them and can`t see how it would be practical for such a design.
If there were supports on the outer wall they would get in the way when the bridge was dropped. Without supports there would be a tremendous stress on the 2/3 of the bridge which was hanging in thin air especially at the stone arch/fulcrum point.
The other 1/3 would`ve needed counter weights underneath it to aid levelling the bridge after it was dropped, and pulleys would be needed below the level of the bridge with ropes going vertical to the underside of the 1/3 in order to pull down on it so raising the 2/3 section.
In theory I can understand the idea, its far quicker to use gravity to drop a bridge into a ditch than it is to manually wind up a drawbridge, to deny access to an attacking force. But if the attackers managed to get into the ditch, the design provides an access ramp albeit at around 30-40 degrees, up which they could clamber, drop down 12-15 feet at the far end and they`re inside the fort. Difficult, but a lot easier than trying to climb vertical walls.
It might be worth asking the person who told you it was a drop down bridge where they got the idea, if its bonafide then there must be some sort of documented evidence as to the design.
Could you keep us informed of any progress you make on this please ? :salut:
Thank you for your reply and interest, Slick :)

First, I have messaged the chap who told me how the bridge worked and I will let you know what he says.

I did do a course on mechanics as part of my Army apprenticeship and can remember things like 'fulcrums', 'levers', and 'pulleys' and stuff, but its really all too vague - although I do generally agree with what you've said.

The proposed method of operation just doesn't 'feel right', it's just too crude - and wouldn't the bridge get damaged if they relied on gravity? (I did have the politically incorrect thought here of, "Maybe they kept a cage of French prisoners underneath to soften the impact", but then thought it might not be wise to say so).

Anyway, when I was a kid the drawbridge in the photo below - a scaled down version of that guarding the North Entrance road tunnel - was pretty much intact (not like the relic you see here):
[align=center]

[Click on the photo to see the original web-page][/align]

All it took was a couple of 5 year-olds on each of the counterweights (out of shot and set into the tunnel floor) and we could make that thing go up and down all day. Not only that, I was always impressed by how smoothly it worked - as I said before, they're perfectly balanced machines "requiring the minimum of effort to operate."

It'll be interesting to see if anything more can be found out about the drop-down bridge - I'll wait to hear from this chap and then maybe go to the local library to see if they've got anything.

I've seen a few pictures of the Plymouth Forts - I wouldn't have minded having them at the top of my backgarden when I was a kid, either!

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#6
I`ve just googled "tenaille" in regard to forts and the descriptions I`ve come across are "a half height outwork protected on the flanks by bastions" and "a landward defence system in place of a moat". Not sure how those fit into the bridge area in question.
Be interesting to know what your contact comes up with in relation to the bridge design.
 
#7
I think I`ve solved the puzzle. Using Wiki and The Dover Western Heights Preservation Society websites I`ve got a better idea of where the bridges are situated in the overall layout.
The two bridges crossed the twin ditches (or lines), which were separated by an earth bank – the tenaille. Neither bridge was fixed. The first of the two had a drop-down section hinged at the tenaille end, while the second had a section that could be raised.
The Trenaille is an outer defence surrounded by moats. Regarding the two bridges, the inner as can be seen in one of the pics above is a drawbridge stretching approximately 1/3 of the distance across the moat, the other 2/3 was a fixed bridge (3) stretching from the Trenaille to the stone arch.
The outer bridge which is the puzzler is similar but the 1/3 section drops into the moat with the hinge on the Trenaille end as opposed to being raised . The 2/3 section here (2) was also fixed and stretched from the outer road to the stone arch.
So the term drop bridge is the opposite to drawbridge, quite sensible as it avoided the need for a high structure being built on the Trenaille in order to raise the bridge, as is evident on the inner drawbridge.
I`m presuming, without being able to take measurements, that the drop section would`ve ended up being vertical and resting against the wall of the Trenaille when the fort was under attack.
A pic from the DWHPS:


Excuse my rather simple drawing :oops:
 
#8
Fine for sudden defence, but not so good for getting out again.

I'm thinking that your 'hinge' would actually be an axle, with the red element being mirrored the other side. This would form part of the roadway on the defenders' side. If placed sweetly it could even come flush with the wall.

With ropes/chains fitted it would make it easier to control the drop/raise - and also provides extra protection.
 
#9
Whiffler thats an excellent idea, never even crossed my mind. The mirrored part would act as a counterweight to help raise the drop part, as well as maybe providing some cover for riflemen to shoot out at the attacking force. Be interesting to see if there is any evidence of a support for an axle in the wall of the Trenaille.
 
#11
Thank you both, whiffler and slick - that's excellent stuff :)

Even before I first posted the videos on a local Dover forum, I had always assumed that only the last 1/3rd of the bridge 'moved'.

Aware that it was only an assumption on my part, however, I was still surprised by the 3rd reply in the Dover forum thread when a chap called 'doverpast' wrote:

The outer bridge was a drop bridge as mentioned. It pivoted about 1/4 the way along it's length. The 3/4 dropped into the ditch and the other 1/4 would almost of create a 'shield' as it went up. It was replaced around WW2 with the permanent girders. It was thought that it was only for a proper emergency as getting the section back up would be hard work !!
This is the chap I've asked for clarification from (with regards to where he got his info) because he not only has his own doverpast website, he is also a member of the Western Heights Preservation Society himself (see here).

The pulleys I mentioned before are now covered by ivy and stuff. I did think about walking out on the bridge a few feet, but I, um, bottled out :oops: (at that point, I wasn't even sure I could get off the tenaille, yet alone taking any additional risks)

If my nerve comes back the next time I'm on the tenaille, then I'll try and locate exactly where the pulleys are (and, if I haven't already fallen off of the bridge, how deep the recess is behind them).

Thanks again - between you, you've come up with the kind of 'elegant' explanation that intuition says makes sense!

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#12
whiffler said:
I shall reward myself with another pint later.
Just what I`m doing now :D

Jorolat I`m not surprised the initial suggestion came from a "society" member. I`ve met a few and they can seem rather odd. During an open day a few years back at Crownhill fort I was chatting to one whilst looking out through one of the bastion ports. I mentioned that I could just imagine a bunch of Johnny French storming down the ditch and giving them a volley out through the ports, he stood there looking none too amused, however it brought a smile to the face of a young Army cadet who was stood in the vicinity. :D
 
#13
slick said:
whiffler said:
I shall reward myself with another pint later.
Just what I`m doing now :D

Jorolat I`m not surprised the initial suggestion came from a "society" member. I`ve met a few and they can seem rather odd. During an open day a few years back at Crownhill fort I was chatting to one whilst looking out through one of the bastion ports. I mentioned that I could just imagine a bunch of Johnny French storming down the ditch and giving them a volley out through the ports, he stood there looking none too amused, however it brought a smile to the face of a young Army cadet who was stood in the vicinity. :D
I haven't met the doverpast chappy at all, but I'm pretty sure I did meet some of the others at a Drop Redoubt open day two years ago.

If so, then the ordinary 'rank and file' were great, but there seemed to be an awful rigidity at the top of the hierarchy, part of which was a proprietary attitude towards both the location and to any knowledge of it (and also towards the 'rank and file' members themselves).

At that time, I had been considering volunteering to help with the conservation work, but I knew I just wouldn't have fitted in - shame, really!

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#14
This is the question I asked the doverpast chappy:

I'm not an engineer or anything, but by comparison, the way the outer drop-down bridge was supposed to work seems remarkably crude - could you tell me where you got the info about how it worked from, please?
To which he has replied:

Info was somewhere in the national archive, very crude but only designed for real emergencies that never came....
I was half-expecting to get an answer like that - I just don't think he's right at all.

I've found this old photo of the bridge which appeared in a Kent Archaeological Review article in the Autumn of 1973:
[align=center]
[/align]


You can see the pulleys I'm on about in the centre of the brick buttressess (if that's the right word - columns?). The pulley on the right-hand side is particularly noticeable, the top of the pulley being level with the second railing down from the top on the bridge.

You can also see how the bridge extends 'into' the tenaille, past the front face of the buttresses, supporting the idea of a mirrored section pivoting at the face of the buttresses.

I'm not sure if the mirrored part could be as long as the drop-down part because of the curve in the cutting, but there again, there's no reason it has to be.

The bridge in the photo is not the original bridge, of course. I think there's a significance in the fact that the width of the brick arch appears to equal the distance between the left edge of the left buttress and the right edge of the right butttress (I'm sure I could have expressed that better, but I haven't had breakfast yet).

Also, there could have been something on either side of the brick arch extending above the bridge level - I'm just intrigued by the pulleys and their role in the great scheme of things.

I mean, I can see how cables from the pulleys might be attached to the drop-down part at the brick-arch end, but they just seem to be in-and-out thingies.

Any ideas (or further thoughts), anyone?

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#15
Thanks for the latest pic, it certainly gives a clearer view of the set up. Although I`m more stumped than ever now as I tend to agree with you that those pulleys don`t seem to be man enough or in a suitable position to raise and lower a section of bridge, especially as the far end of the section wouldn`t be supported. They also seem to be in a very vulnerable position to operate, but again I can`t see any operating system ?
There does seem to be a large recess in the wall under the bridge into which the bridge section could be dropped, but this is purely speculation.
I did consider a sliding section which could be retracted into the Tenaille but given the curve of the road I would think this is highly improbable, as is a mirrored section of similar length as the bridge. Any mirrored section would have to be lower than the caps on the pulley pillars or else it would foul the caps when raised.
The width of the arch in relation to the pulley buttresses could be a crucial point, it could be surmised that the original bridge was at least as wide as the arch which might suggest that the movable section would extend to the outer edges of the pulley pillars.
I think maybe a close look at the pulleys and anything else in the close vicinity to try and work out an operating sytem might be the next step. Whether the bridge was raised or dropped it would need something a bit more substantial than just the two pulleys I would imagine.
I`m getting a gut feeling that the drop bridge isn`t a particularly good idea as its not supported at one end. If the bridge was under constant use then this would put a tremendous strain on the ropes/chains needed to support it which in turn would mean an undue amount of maintenance/checking, possible daily.
I sometimes get frustrated at living in the arrse of the country as I wouldn`t mind a trip to this place to get a first hand look, although I`d probably bring a strimmer and machete with me. :D
 
#18
slick said:
Cheers, will look forward to the pics :)
Hi Slick,

I feel 'proof positive' of how the outer bridge worked lies in the third of the images, but by way of preamble I went up the Western Heights on Sunday and as we went under the bridge I took this photo:

[align=center]

[Click on the photo to see the original web-page][/align]

As you can see there is only one horizontal layer of original bricks above the three layers forming the curved archway.

If you were to click on the the photo below, and then click again on the next photo displayed (to see a larger version), you would see there are five horizontal layers of original bricks above the archway - indicating the brick arch sloped towards the tenaille, thereby allowing an arc of movement for a drop-down bridge:

[align=center]

[Click on the photo to see the original web-page][/align]

I had intended to go up there sometime this week for more evidence (I didn't have time on Sunday), but it isn't necessary now because I went to a Kent History forum topic and found this image of the original plans:

[align=center]
[/align]

Amazing what you can find on the internet! :)

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 
#19
Ah, all explained then, and Whiffler was correct about the mirror section which would`ve formed a "wall" when the bridge was dropped. Looking at the design would it be safe to say that a lot of the metalwork in the photos is original, especially the uprights ?
Would there have been any support at the far end of the drop section (red arrow) or would it have just been suspended on the ropes ?
 
#20
Whiffler was indeed correct!

I'm pretty certain that all the uprights and 'cross-wires' are original and only the I-beams and concrete block on top of the brick arch are additions.

Probably like you, I don't think it would have been just suspended on cables or ropes, though. In the side-view of the plan you can see the cables going to the bottom of the drop-down section. I sort of imagine something there like the equivalent of a hinged flap so that an initial lifting of the drop-down section causes the flap to drop away thereby enabling the section to swing down against the moat wall.

Having said that, as the drop-down part starts approaching anything like the vertical, it would put an increasingly greater stain upon the dinky little pulleys.

Looking at the plan view of the bridge, I'm wondering if there were pulleys (or an axle, maybe) on the brick arch, too, because that would make the raising and lowering of the bridge a whole lot easier - ie, in the 'down' position, the cables would go from the pulleys to the brick arch and then to the end of the drop-down section lying flat against the bottom of the moat wall.

Pure speculation, of course, but good fun!

Having a plan of something doesn't necessarily mean that was how it was actually built. What also intrigues me about the plan given here is that in the side elevation you can see the cable from the pulley trailing off to the left at an angle of maybe 20 degrees below the horizontal.

To me, this implies they go into the tenaille which sure would be interesting :)

Finally, while I'm here I may as well plug a 3-part video of the abandoned North Centre Bastion, which is this place here:

[align=center]

[Click on the photo to see the original web-page][/align]

John Latter / Jorolat

Images of Dover | My Facebook | My YouTube | Evopsychology.com | Dover Blog
 

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